1. Field of the Invention
The present invention relates to a coordinate reading device.
2. Description of Related Art
Various types of coordinate reading device have been proposed. A representative example of the conventional coordinate reading device is disclosed in Japanese Patent Application Publication (Kokai) No. HEI-5-165560.
FIG. 1(a) is a configuration of a conceivable coordinate reading device. The conceivable coordinate reading device includes: a tablet 91, a scanning circuit 92, and a detection circuit 90. The tablet 91 is provided with a plurality of X sense coils (conductive loop wires) X1 to Xm and Y sense coils (conductive loop wires) Y1 to Yn. The X sense coils X1 to Xm are for detecting X coordinates of a pen (target member) 400, the Y sense coils Y1 to Yn are for detecting Y coordinates of the pen 400. The scan circuit 90 is for serially scanning the sense coils X1-Xm and Y1-Yn on the tablet 91. The detection circuit 90 is for calculating the x and y coordinates of the pen by detecting induction signals generated at the sense coils X1-Xm and Y1-Yn.
The pen 400 includes a coil 401 that generates an alternating magnetic field. When the pen 400 contacts the tablet 91, some sense coils that are located near to the pen 400 generate induction signals 97 due to magnetic coupling with the alternating magnetic field from the coil 401. The induction signals 97 are inputted into the detection circuit 90. The induction signals 97 are amplified at an amplifier 93 and then its amplitude is detected at an amplitude detection circuit 94. Next, an A/D conversion circuit 95 converts the measured amplitude into digital values, and outputs the digital values to a CPU 96. The CPU 96 calculates the positional coordinate of the pen 400 based on the inputted digital values from the A/D conversion circuit 95. For example, the CPU 96 may refer to a coordinate table that stores data indicative of a relationship between a plurality of digital values and coordinate positions. The CPU 96 selects a positional coordinate that corresponds to the digital values presently inputted from the A/D conversion circuit 95.
According to the above-described conceivable coordinate reading device, the coordinate table stores fixed digital values in correspondence with the coordinate positions. It is noted, however, that when the batteries inside the pen 400 are get used up, the output level of the alternating magnetic field drops. In this case, the sense coils will generate induction signals whose levels correspond to the thus decreased output level of the alternating magnetic field. If the CPU 96 detects such an induction signal, and selects the position coordinate in correspondence with the amplitude of the induction signal, accuracy in the coordinate reading is reduced.
FIG. 1(b) illustrates how the precision of coordinate reading drops. As shown in FIG. 1(b), when the battery is fresh, an output voltage V1 will be detected when the pen is at coordinate P1. However, once the battery is get used up, even when the pen is located at the same place P1, the voltage V2, that is lower than the original voltage V1, will be detected, and therefore the pen coordinate will be detected as coordinate P2. This results in a reading error of xcex94P(=P2xe2x88x92P1).
Japanese Patent-Application Publication (Kokai) No. HEI-7-56677 discloses a method for correcting coordinate tables depending on such drops in voltage. FIG. 1(c) is a graph representing the relationship between a position of the pen and a voltage generated at one sense coil. In the method disclosed by the publication, a maximum voltage value Va, generated by one sense coil, is detected, and is compared with a prestored reference value. The coordinate table is then corrected based on the results of this comparison.
However, as apparent from the graph of FIG. 1(c), the plot of voltage change is almost flat in the vicinity of the maximum value Va. Therefore, it is difficult to detect the true maximum value Va accurately. For this reason, it is difficult to correct the coordinate table with a great deal of precision.
It is therefore an objective of the present invention to provide an improved coordinate reading device that is capable of reading correct coordinates of the target member with high precision.
In order to attain the above and other objects, the present invention provides a coordinate reading device for reading a coordinate of a position of a target member, which generates an alternating magnetic field, the coordinate reading device comprising: a coordinate input sheet that has a coordinate input surface defining a coordinate of a target member that generates an alternating magnetic field; a loop-wire attaching member that is provided below the coordinate input sheet and that has a plurality of loop wires, each loop wire being capable of producing a signal in response to the alternating magnetic field; a detection unit that detects the signals from the plurality of loop wires; a coordinate determination unit that determines a coordinate of the position of the target member based on the signal from at least one of the loop wires that is selected for coordinate determination; and a correction unit that controls the coordinate determination unit to determine a corrected coordinate of the position of the target member based on a relationship between a predetermined reference value and the value of the signal from at least one of the plurality of loop wires that is selected for coordinate correction.
The plurality of loop wires may include one correction loop wire that is located on a predetermined position on the loop-wire attaching member and that is used exclusively for the correction operation, wherein the correction unit compares the value of the signal from the correction loop wire with the predetermined reference value, and controls the coordinate determination unit to determine the corrected coordinate of the position of the target member based on the compared result.
The correction unit may include: a selection unit that selects, for coordinate correction, at least two loop wires among the plurality of loop wires, based on the relationship between the values of the signals from the at least two loop wires; a ratio calculation unit that calculates a ratio between the value of the signal from at least one loop wire in the at least two loop wires selected for coordinate correction and the predetermined reference value; and a control unit that controls the coordinate determination unit to determine the corrected coordinate of the position of the target member based on the calculated ratio.
The selection unit may select two loop wires that are adjacent to each other and that produce the signals of the same values with each other, and wherein the ratio calculation unit calculates a ratio between the value of the signal from one loop wire in the selected two loop wires and the predetermined reference value.
The selection unit may select one maximum loop wire that produces the signal of the maximum values among all the loop wires and two adjacent loop wires that are adjacent to the one loop wire. In this case, the selection unit may include a judging unit that judges whether the selected two adjacent loop wires produce the signals of the same values, the ratio calculation unit calculating a ratio between the value of the maximum loop wire and the predetermined reference value when the two adjacent loop wires produce the signals of the same values. Or, the selection unit may include a judging unit that judges whether one of the selected two adjacent loop wires produces the signal of the value that is lower than a predetermined threshold value, the ratio calculation unit calculating a ratio between the value of either one of the maximum loop wire and the other one of the adjacent loop wire and the predetermined reference value when the one of the two adjacent loop wires produces the signal of the value that is lower than the predetermined threshold value.
The selection unit may select one loop wire and two adjacent loop wires that are adjacent to the one loop wire and that produce the signals of the same values, the ratio calculation unit calculating a ratio between the value of the one loop wire that is sandwiched between the two adjacent loop wires and the predetermined reference value when the two adjacent loop wires produce the signals of the same values.
According to another aspect, the present invention provides a coordinate reading device for reading a coordinate of a position of a target member, which generates an alternating magnetic field, the coordinate reading device comprising: a coordinate input sheet that has a coordinate input surface defining a coordinate of a target member, which employs a battery and which generates an alternating magnetic field whose strength corresponds to a condition of the battery; a loop-wire attaching member that is provided below the coordinate input sheet and that has a plurality of loop wires, each loop wire being capable of producing a signal in response to the alternating magnetic field, the signal having a value that corresponds to the location of the target member and to the condition of the battery in the target member; a detection unit that detects the signals from the plurality of loop wires; a coordinate determination unit that determines a coordinate of the position of the target member based on the signal from at least one of the loop wires that is selected for coordinate determination; and a correction unit that controls the coordinate determination unit to determine a corrected coordinate of the position of the target member dependently on the condition of the battery in the target member.